The present invention relates to an ink jet printing apparatus which may be utilized in a facsimile transceiver, copying machine, oscillographic or chart recorder or the like.
In such an apparatus an ink ejection printing head is moved relative to a sheet of printing paper and ink is ejected from the head onto the paper. An electrostatic charge is applied to the ejected ink which passes between electrostatic deflection electrodes prior to impact with the paper. An electrical deflection signal applied to the deflection electrodes causes the jet of ink to be deflected to a desired degree to form an oscillographic trace, characters, etc.
The performance of the apparatus is increased tremendously if the ink is atomized in the ejection head. In order to achieve this effect, it has been known to dispose an ultrasonic vibrator in the head. The vibrator pressure or velocity modulates the ink and thereby atomizes the same.
However, such ultrasonic vibrators must be rather small in size for adequate mechanical response at the relatively high frequencies, such as 100 KHz, required for atomization of ink. The amplitude of oscillation, and therefore the displacement of ink produced by such a vibrator is generally inadequate to produce effective atomization. Increasing the size of the vibrator enables greater amplitude of oscillation but reduces the mechanical response of the vibrator at high frequencies to such an extent that a disproportionate amount of drive power is required to produce oscillation.
A prior art attempt to overcome this problem is to match the drive frequency to the mechanical resonance frequency of the head. However, such a match is unobtainable due to the large size of the head.
Another prior art expedient is to provide a plurality of small ultrasonic vibrators which are driven in sets. This generally overcomes the problems involved since the small vibrators may be driven at high frequency and mechanically connected in parallel so that the oscillation amplitudes are summed. However, a major problem remains even in this system in that any harmonics or distortions in the waveform of the drive signal have a major degrading effect on the mechanical response and therefore oscillation efficiency of the vibrators. A satisfactory means of eliminating such distortions has not been heretofore devised.